X-ray computed tomography (CT) scanners have been used in security screening in airports for several years. A conventional system comprises an X-ray tube that is rotated about an axis with an arcuate X-ray detector which is also rotated, at the same speed, around the same axis. The conveyor belt on which the baggage is carried is placed within a suitable aperture around the central axis of rotation, and moved along the axis as the tube is rotated. A fan beam of X-radiation passes from the source through the object to be inspected and subsequently to the X-ray detector array.
The X-ray detector array records the intensity of X-rays passed through the object to be inspected at several locations along its length. One set of projection data is recorded at each of a number of source angles. From these recorded X-ray intensities, it is possible to form a tomographic (cross-sectional) image, typically by means of a filtered back projection algorithm. In order to produce an accurate tomographic image of an object, such as a bag or package, there is a requirement that the X-ray source pass through every plane through the object. In the arrangement described above, this is achieved by the rotational scanning of the X-ray source, and the longitudinal motion of the conveyor on which the object is carried.
In this type of system the rate at which X-ray tomographic scans can be collected is dependent on the speed of rotation of the gantry that holds the X-ray source and detector array. In a modern CT gantry, the entire tube-detector assembly and gantry will complete two to four revolutions per second. This allows up to four or eight tomographic scans to be collected per second, respectively.
As the state-of-the-art has developed, the single ring of X-ray detectors has been replaced by multiple rings of detectors. This allows many slices (typically 8) to be scanned simultaneously and reconstructed using filtered back projection methods adapted from the single scan machines. With a continuous movement of the conveyor through the imaging system, the source describes a helical scanning motion about the object. This allows a more sophisticated cone-beam image reconstruction method to be applied that can in principle offer a more accurate volume image reconstruction.
However, rotating gantry X-ray inspection systems are expensive to install, have a large footprint and consume a lot of power.
Some conventional CT scanners comprise non-rotating stationary gantry systems, which project X-ray beams from fixed, stationary sources at the subjects to be scanned. These systems include one or more spatially distributed X-ray sources for emitting X-rays and one or more X-ray detectors for detecting the X-rays. Multiple X-ray sources are required to be activated at the same time to produce a fan beam of X-rays in order to create a three-dimensional scanned image of an object. Stationary gantry systems may use anywhere from a dozen to a few hundred X-ray sources to produce a scanned image that varies in quality depending on the number of X-ray sources used. However, increasing the number of sources adds complexity to the designs of scanning systems and also increases their cost of manufacturing as well as operation. Additionally, traditional stationary gantry systems consume high amounts of power and are difficult to maintain.
Hence, what is needed is an improved X-ray inspection system that is efficient in detecting threat materials, is less expensive, has a smaller footprint and may be operated using regular line voltage power.